1. Field of the Invention
This invention relates to an engine speed detecting system for a multiple-displacement engine in which operating state of the engine can be changed between a full-displacement state in which all the cylinders output power and a part-displacement state in which a part of the cylinders are cut out or are made inoperative according to the operating condition of the engine.
2. Description of the Prior Art
In order to improve the fuel consumption rate, there has been proposed a multiple-displacement engine in which operating state of the engine can be changed between the full-displacement operation and the part-displacement operation according to the operating condition of the engine. See Japanese Unexamined Patent Publication No. 57(1982)-338. For example, when the engine operates under heavy load, e.g., when the vehicle starts or runs at a high speed, fuel is supplied to all the cylinders so that all the cylinders contribute to power output, while when the engine operates under relatively light load, e.g., when the vehicle cruises at a steady speed, fuel supply to a part of the cylinders is cut off to make inoperative the cylinders and to improve the volumetric efficiency of the remaining cylinders, thereby providing saving in fuel.
Such multiple-displacement engine is provided with a cylinder number control means for cutting fuel supply to a part of the cylinders to change the operating state of the engine to the part-displacement operation, and a cylinder reduction determination means which determines whether or not the operating state of the engine is to be changed to the part-displacement operation according to the operating conditions of the engine such as engine speed, throttle position, intake manifold vacuum and coolant temperature, and delivers the determination to the cylinder number control means.
Engine speed or engine rpm is often detected in order to control the engine, for example to control the idling speed, and to control the amount of fuel to be injected. The engine speed can be determined on the basis of the time required for the crankshaft to rotate by a predetermined angle or one period.
However, when the engine speed detection is simply applied to the multiple-displacement engine, the engine speed cannot be precisely detected in the part-displacement operation of the engine. That is, when the engine operates in the full-displacement state, the time required for the crankshaft to rotate by a predetermined crank angle hardly fluctuates for a given engine speed. However, when the engine operates in the part-displacement state the predetermined crank angle sometimes includes a power stroke and sometimes does not include a power stroke. Accordingly, the time required for the crankshaft to rotate by the predetermined angle fluctuates and engine speed detection becomes inexact.